he US is a major supplier of food and fiber for the world,accounting for more

than 25% of the total global trade in wheat,corn,soybeans,and cotton.

Cropland currently occupies about 400 million acres,or 17% of the total US land

area. In addition, grasslands,and permanent grazing and pasturelands,occupy

almost 600 million acres,another 26% of US land area. The value of agricultural

commodities (food and fiber) exceeds $165 billion at the farm level and over $500

billion,10% of GDP, after processing and marketing.

Economic viability and competitiveness are major concerns for producers trying to

maintain profitability as real commodity prices have fallen by about two-thirds over

the last 50 years. Agricultural productivity has improved at over 1% per year since

1950,resulting in a decline in both production costs and prices. This trend maintains intense pressure on individual producers to continue to increase the productivity of their farms and to reduce costs of production. In this competitive economic environment,producers see anything that might increase costs or limit their markets as a threat to their viability. Issues of concern include regulatory actions that

might increase costs,such as efforts to control the off-site consequences of soil erosion, agricultural chemicals,and livestock wastes; growing resistance to and restrictions on the use of genetically modified crops; extreme weather or climate events

such as droughts and floods;new pests;and the development of pest resistance to

existing pest control strategies. Future changes in climate will interact with all of

these factors.

The agriculture sector Assessment considered crop agriculture, grazing,livestock,

and environmental effects of agriculture. The focus in this document is primarily on

crop agriculture which was studied most intensively in this Assessment. Although

extensive,the analysis of crop yields did not fully consider all of the consequences

of possible changes in pests,diseases,insects,and extreme events resulting from climate change. This analysis assumes continued technological advances and no

changes in federal policies or international trade.

Key Issue: Crop Yield Changes and Associated 

Economic Consequences

t is likely that climate change,as defined by the scenarios examined in this

Assessment,will not imperil the ability of the US to feed its population and to

export foodstuffs. Results of this Assessment suggest that,at the national level,productivity of many major crops will likely increase under the climate scenarios used

in these crop models. Crops showing generally positive results include cotton,corn

for grain and silage,soybeans,sorghum,barley,sugar beets,and citrus fruits.

Pastures also show positive results.

KEY ISSUES

• Crop Yield Changes and

Associated Economic

Consequences

• Changing Water

Demands for Irrigation

• Surface Water Quality

• Increasing Pesticide Use

• Climate Variability

CO2 Effects on Crops

Greater concentrations of CO2 generally

result in higher photosynthesis rates and

may also reduce water losses from plants.

Photosynthesis is enhanced when additional

carbon is available for assimilation and so

crop yields generally rise.

The actual response to increased CO2 differs

among crops. Most commercial crops in the

US, including wheat, rice, barley, oats, potatoes, and most vegetable crops, tend to

respond favorably to increased CO2, with a

doubling of atmospheric CO2concentration

leading to yield increases in the range of 15-

20%. The crop models used in this

Assessment assume a CO2fertilization effect

in this range, and also assume that sufficient

nutrients and water will be available to support these increases. Other crops including

corn, sorghum, sugar cane, and many tropical grasses, are less responsive to increases

in CO2, with a doubling of its concentration

leading to yield increases of about 5%. 

In situations where crop yields are severely

limited by factors such as nutrient availability, an enduring CO2

fertilization effect is very

likely to be of only minor importance.

I

AGRICULTURE SECTOR

T

Economically, consumers

are likely to benefit from

lower prices while producers are likely to see their

profits decline.

90
91

For other crops,including wheat, rice,oats,hay,sugar cane,potatoes,and tomatoes,yields are projected to increase under some conditions and decrease

under others. The crop models assume that the CO2fertilization effect will be

considerable (see box).

In the crop yield models,a limited set of on-farm adaptation options are considered,including changes in planting dates and changes in varieties. These contribute small additional gains in yields of dryland crops and greater gains in

yields of irrigated crops. The economic models consider a far wider range of

adaptations in response to changing productivity, prices,and resource use,

including changes in crops and the location of cropping,irrigation,use of fertilizer and pesticides,and a variety of other farm management options.

All agricultural regions of the US are not affected to the same degree by the climate scenarios studied in this Assessment. In general,this study finds that climate change favors northern areas. The Midwest,West,and Pacific Northwest

exhibit large gains in yields with both climate scenarios in the 2030 and 2090

time frames. Crop yield changes in other regions vary more widely depending

on the climate scenario and time period. For example,projected wheat yields

in western Kansas decline under the Canadian scenario.

Dryland Yields in 2030 - 2090

Without Adaptation

Dryland Yields in 2030 - 2090

With Adaptation

...at the national level, productivity of many major crops

will likely increase under the

climate-change scenarios

used in the crop models.

Crops showing generally

positive results include cotton, corn for grain and silage,

soybeans, sorghum, barley,

sugar beets, and citrus fruits. 

-40 -20 0 20 40 60 80 100 120 140

Percent

Cotton

Corn

Soybean

Spring wheat

Winter wheat

Sorghum

Rice

Barley

Oats

Hay

Sugar cane

Sugar beets

Potato

Orange

Pasture

Grapefruit

Canadian Scenario 2030

Canadian Scenario 2090

Hadley Scenario 2090

Hadley Scenario 2030

-50 0 50 100 150 200

Percent

Corn

Rice

Hay

Canadian Scenario 2030

Canadian Scenario 2090

Hadley Scenario 2090

Hadley Scenario 2030

Model simulations of average changes in crop yields for 16 crops. The yield changes are given as percentages and represent the differences between current yields and those projected for two time periods, 2030 and 2090. Two scenarios of future climate, the Canadian

and Hadley, were used. The results consider physiological responses of the crops to climate under either dryland or irrigated cultivation.

They also consider either "no adaptation" or "adaptation" responses by producers to climate change. Adaptations included changes in

planting dates and crop varieties. Only 11 of the 16 crops were actually modeled: cotton, wheat (winter and summer), corn, hay, potato,

orange, soybean, sorghum, rice, pasture grass. Results for the other crops are based on extrapolations from the modeled crops.
-40 -20 0 20 40 60 80 100 120 140 Percent

Cotton

Corn

Soybean

Spring wheat Winter wheat Sorghum

Rice Barley

Oats

Hay

Sugar cane

Sugar beets Potato

Orange

Pasture

Grapefruit

Canadian Scenario 2030

Canadian Scenario 2090

Hadley Scenario 2090

Hadley Scenario 2030

Model simulations suggest that the net effects of the climate scenarios studied on the agricultural segment of the

US economy over the 21st century are generally positive.

The exceptions are simulations under the Canadian scenario in the 2030 time period,particularly in the absence

of adaptation.

Economically, consumers benefit from lower prices while

producers’profits decline. Under the Canadian scenario,

these opposing economic effects are nearly balanced,

resulting in a small net effect on the national economy.

The estimated $4-5 billion reduction in producers' profits

represents a 13-17% loss of income,while the savings of

$3-6 billion to consumers represent less than a 1% reduction in the consumers' food and fiber expenditures. This

large difference exists because much of the final cost of

agricultural goods to consumers reflects processing,transportation,and retailing costs that the models used here

assume are not affected by climate. Under the Hadley scenario,producers' profits decline by up to $3 billion (10%),

while consumers save $9-12 billion (in the range of 1%).

The major difference between the model outputs is that

under the Hadley scenario,productivity increases are substantially greater than under the Canadian,resulting in

lower food prices,to the consumers' greater benefit. The

smaller producer losses in the Hadley scenario,despite

greater productivity gains and price changes,reflect the

fact that the US farmers' advantage over foreign competitors grows and they are thus able to significantly increase

export volume. Analyses show that producer versus consumer effects depend on how climate change affects production elsewhere in the world. The sector Assessment

was not able to extend its estimates on crop and livestock

production to other regions of the world but used worldwide shifts in crop and livestock production projected in

previous studies.

Regional production change,the total value of crop and

livestock production,is positive for all regions in both the

2030 and 2090 time frames under the Hadley scenario.

Adaptation measures have a small additional positive

effect. In contrast,this economic index differs among

regions under the Canadian scenario in both the 2030s

and 2090s. It is positive for most northern regions,mixed

for the northern Plains,and negative for Appalachia,the

Southeast,the Delta states,and the southern Plains.

Adaptation measures help somewhat for the southern

regions,but the value of production is lower in these

regions under both the 2030 and 2090 climates considered.

Irrigated Yields in 2030 - 2090

Without Adaptation

Irrigated Yields in 2030 - 2090

With Adaptation

-100 -50 0 50 100 150 Percent

Cotton

Corn

Soybean

Spring wheat Winter wheat Sorghum

Rice Barley

Oats

Hay

Sugar cane

Sugar beets Potato

Orange

Grapefruit Pasture

Canadian Scenario 2030

Canadian Scenario 2090

Hadley Scenario 2090

Hadley Scenario 2030

AGRICULTURE KEY ISSUES

92
93

Changing Water Demands 

for Irrigation

t the national level,the models

used in this Assessment find

that irrigated agriculture’s need for

water declines approximately 5-10%

for 2030,and 30-40% for 2090 in the

context of the two primary climate

scenarios. At least two factors are

responsible for this possible reduction. One is increased precipitation

in some agricultural areas. The other

is that faster development of crops

due to higher temperatures results in

a reduced growing period and thereby reduced water demand. In the

crop modeling analyses done for this

Assessment,shortening of the growing period reduces plant water-use

enough to more than compensate for

the increased water losses from

plants and soils due to higher temperatures.

The picture for future agricultural

water demands at the regional scale

is less clear and it is possible that it

will differ substantially from the

national picture. At the regional level,

there is the possibility that overall

water use will increase in response to

climate change.

Regional Production Changes Relative to Cur rent Production

2030 and 2090 periods

4

3

2

1

0

-1

14

12

10

8

6

4

2

0

U.S. Total Surplus

Economic Impacts of climate change under the Canadian and Hadley climates. The economic index is change in welfare

expressed as the sum of producer and consumer surplus in billions of dollars. US (light blue bar above) includes sales

and purchases in the US, while Total Surplus (dark blue bar) also includes overseas sales by US producers. 

-80

-60

-40

-20

0

20

40

60

80

100

0

20

40

60

80

100

120

140

Canadian Model

Canadian Model

Hadley Model

Hadley Model

Regional production change (crop and

livestock production weighted by prices)

from a year 2000 baseline was positive

for all regions in both the 2030 and 2090

timeframes under the Hadley scenario.

In contrast this index differed among

regions under the Canadian scenario in

both the 2030s and 2090s. It was positive for most northern regions, mixed for

the northern Plains, and negative for

Appalachia, the Southeast, the Delta

states and the southern Plains.

Economic Impacts of Climate Change on US Agriculture

2030 and 2090 periods

A

Model simulations suggest that the net effects

of climate change on

the agricultural segment

of the US economy

over the 21st century

are generally positive.

2030 2030 w/adaptation 2090 2090 w/adaptation
94

Surface Water Quality

case study of agriculture in the

drainage basin of the

Chesapeake Bay was undertaken to

analyze the effects of climate change

on surface-water quality. The Bay is a

highly valuable natural resource that

has been severely degraded in recent

decades. Soil erosion and excess

nutrient runoff from crop and livestock production have played a major

role in the decline of the Bay's health.

In simulations for this Assessment,

under the two climate scenarios for

2030,loading of excess nitrogen into

the Chesapeake Bay due to corn production increases by 17-31% compared with the current situation.

These projected effects may not fully

represent the effects of extreme

weather events such as floods or

heavy downpours that wash large

amounts of fertilizers and animal

manure into surface waters. Changes

in future farm practices,such as better matching of the timing of plant

need for fertilizer with the timing of

application,could possibly help to

reduce the projected impacts.

Because efforts are already underway

to protect the Bay, many of these

practices may be required and in use

before 2030.

Pesticide Use

he Assessment investigates the

relationship between pesticide

use and climate for crops that require

relatively large amounts of pesticide.

Pesticide use is projected to increase

for most crops studied and in most

states,under the climate scenarios

considered. Increased need for pesticide application on corn is generally

in the range of 10-20%,on potatoes,5-

15%,and on soybeans and cotton,2-

5%. The results for wheat vary widely

by state and climate scenario showing changes in pesticide application

ranging from approximately –15 to

+15%.

The increase in pesticide use results

in slightly poorer overall economic

performance,but this effect is quite

small because pesticide expenditures

are a relatively small share of production costs. This Assessment approach

does not consider increased crop

losses due to pests,implicitly assuming that all additional losses are eliminated through increased pest control

measures. This may underestimate

losses due to pests associated with

climate change.

In addition,this Assessment does not

consider the environmental consequences of increased pesticide use 

and it is possible that these would be

substantial. In a complete economic

analysis,the costs of negative impacts

of pesticides on the environment

would be considered.

Climate Variability

he consequences of climate

change for US agriculture are

very likely to be affected by changes

in climate variability and extreme

events. Agricultural systems are vulnerable to climate extremes,with

effects varying from place to place

because of differences in soils,production systems,and other factors.

Changes in precipitation type (rain,

snow, or hail),timing,frequency, and

intensity, along with changes in wind

(windstorms,hurricanes,and tornadoes),are likely to have significant

consequences. Heavy precipitation

events cause erosion, waterlogging,

and leaching of animal wastes,pesticides, fertilizers,and other chemicals

into surface and groundwater.

A major source of weather variability

is the El Niño Southern Oscillation

(ENSO). ENSO effects vary widely

across the country. Better prediction

of these events would likely allow

farmers to plan ahead,altering their

choices of which crops to plant and

when to plant them. The value of 

AGRICULTURE KEY ISSUES

A

T

T

140

120

100

80

60

40

20

0

1950 1960 1970 1980 1990 2000

Reductions in corn yields often correspond to

extreme climate events including droughts and

floods. The record Midwest floods of 1993 resulted from this being the wettest year on record,

washing out and flooding many corn fields and

resulting in late replanting. In 1995, declines in

yields resulted from a sequence of unusual climate events; a cool wet spring delayed planting,

and a hot, dry summer affected pollination, and

ultimately, yield. 

Corn Yields in US 1950 - 99

(bushels per acre)
95

improved forecasts of ENSO events

under their current intensity and frequency has been estimated at approximately $500 million per year.

As climate warms,ENSO is likely to

be affected. Some models project

that more frequent El Niños and

stronger La Niñas will have increasing

impacts on US weather. The potential

impacts of changes in frequency and

strength of ENSO conditions on agriculture were modeled in this

Assessment. An increase in these conditions is found to cost the US $320

million per year if accurate forecasts

of these events are available and

farmers use them as they plan for the

growing season. The increase in cost

is projected to be greater if accurate

forecasts are not available or not

used.

Adaptation Strategies

daptations such as changing

planting dates and choosing

longer season varieties are likely to

offset losses or further increase

yields. Adaptive measures are likely

to be particularly critical for the

Southeast because of the large reductions in yields projected for some

crops under the more severe climate

scenarios examined. Breeding for

response to CO2 will likely be necessary to achieve the strong fertilization

effect assumed in the crop studies.

This is an unexploited opportunity

and the prospects for selecting for

CO2response are good. However,

attempts to breed for a single characteristic are often not successful,

unless other traits and interactions

are considered. Breeding for tolerance to climatic stress has already

been heavily exploited and varieties

that do best under ideal conditions

usually also outperform other varieties under stress conditions.

Breeding specific varieties for specific

conditions of climate stress is therefore less likely to encounter success.

Some adaptations to climate change

and its impacts can have negative secondary effects. For example,an

examination of use of water from the

Edward’s aquifer region around San

Antonio,Texas found increased pressure on groundwater resources that

would threaten endangered species

dependent on spring flows supported by the aquifer. Another example

relates to agricultural chemical use.

An increase in the use of pesticides

and herbicides is one adaptation to

increased insects, weeds,and diseases

associated with warming. Runoff of

these chemicals into prairie wetlands,

groundwater, and rivers and lakes

could threaten drinking water supplies,coastal waters,recreation areas,

and waterfowl habitat.

The wide uncertainties in climate scenarios, regional variation in climate

effects,and interactions of environment,economics,and farm policy

suggest that there are no simple and

widely applicable adaptation prescriptions. Farmers will need to

adapt broadly to changing conditions

in agriculture,of which changing climate is only one factor. Some of the

possible adaptations more directly

related to climate include:

• Sowing dates and other seasonal

changes:

Plant two crops instead of one or a spring

and fall crop with a short fallow period to

avoid excessive heat and drought in midsummer. For already warm growing

areas, winter cropping could possibly

become more productive than summer

cropping.

• New crop varieties:

The genetic base is very broad for many

crops, and biotechnology offers new

potential for introducing salt tolerance,

pest resistance, and general improve -

ments in crop yield and quality.

• Water supply, irrigation, and drainage

systems:

Technologies and management methods

exist to increase irrigation efficiency and

reduce problems of soil degradation, but

in many areas, the economic incentives to

reduce wasteful practices do not exist.

Increased precipitation and more intense

precipitation will likely mean that some

areas will need to increase their use of

drainage systems to avoid flooding and

water-logging of soils.

• Tillage practices:

A warmer climate will speed the decay of

soil organic matter by bacteria and fungi.

Loss of organic matter reduces the capacity of soils to store water and nutrients

essential for plant growth. Tillage practices that incorporate crop residues in the

soils would likely combat this loss and

improve soil quality.

• Use near-term climate predictions:

Accurate six-month to one-year forecasts

could possibly reduce losses due to

weather variability. For example, predictions of El Niño events have proven useful

in regions where El Niño strongly affects

weather.

• Other management adjustments:

Virtually all components of the farming

system from planting to harvesting to selling might be modified to adjust to climate

change.

A